The present invention relates to a method for greatly improving the efficiency of removal or decontamination of radioactive and/or non-radioactive ruthenium (hereinafter simply referred to as ruthenium) present in a nitric acid solution in an acid recovery system where a waste nitric acid solution discharged from, for example, spent nuclear fuel reprocessing facilities is concentrated by evaporation and condensation to recover nitric acid.
In the reprocessing of spent nuclear fuel, chopped nuclear fuel is dissolved in nitric acid. Nitric acid is used also in an extraction stage where each of uranium, plutonium and fission products is mutually separated and purified.
The nitric acid solution which has been used in the dissolving or extraction stages is reused after reducing the radioactivity (i.e. decontamination) and reconverting them into about 10M nitric acid by generating nitric acid vapor by evaporation of the solution and condensing the nitric acid vapor in an acid recovery system. A nitric acid evaporator in which the nitric acid solution is heated to generate nitric acid vapor by evaporation is generally used in such an acid recovery system.
When ruthenium, irrespective of whether it is radioactive or non-radioactive, is contained in the nitric acid solution, it is oxidized during an evaporation treatment to form a volatile compound (RuO.sub.4) which is then evaporated and contained in nitric acid vapor. As a result, the concentration of ruthenium in the recovered nitric acid is not well reduced, and the decontamination efficiency of ruthenium in the acid recovery system is lowered.
Thus, there have been proposed various methods wherein sodium nitrite, NOx gas, sucrose or formalin is added in order to improve the decontamination efficiency in the nitric acid evaporator.
However, these methods have the following disadvantages. The addition of sodium nitrite increases salt concentration in a concentrated liquid waste within the evaporator and causes troubles in subsequent steps. NOx gas is hard to handle and requires a difficult addition process. Sucrose and formalin decompose nitric acid, so that there must be provided an apparatus for recovering decomposed nitric acid in a evaporated vapor treatment system.